1. Field of the Invention
The present invention relates to a structure of electrical connections between a stepping motor and a circuit board. More particularly, it relates to a structure of electrical connections between a stepping motor and a circuit board suitable for those applications in which the stepping rmLtor is used as a prime mover of an analog instrument for a mobile unit, such as a motor vehicle.
2. Description of the Related Art
Conventionally, a cable terminated with or without a connector has usually been used to establish electrical connections between a stepping motor and a circuit board incorporating a control circuit for driving the stepping motor, for instance. This kind of conventional structure requires complex wiring work and inevitably increases the size of an instrument. There exist previous approaches to the solution of this problem, such as those proposed in Japanese Unexamined Patent Publication Nos. 8-111971 and 5-64411. These previous approaches are schematically depicted in FIG. 6, in which a stepping motor 3 rotatably accommodates a rotor 2 having a drive shaft 1, at least an output end of the drive shaft 1 projecting from a front surface (top side in FIG. 6) of the stepping motor 3, terminals 4 extending parallel to an axial direction of the drive shaft 1 are provided on the stepping motor 3, and circuit board 5 is provided on a rear surface (bottom s 4de in FIG. 6) of the stepping motor 3, opposite to its front surface from which the output end of the drive shaft 1 projects, wherein the stepping motor 3 is directly connected to the circuit board 5 by soldering the stepping motor 3 to the circuit board 5, for instance. This structure makes it possible to connect the stepping motor 3 and the circuit board 5 by using an automatic soldering process on a production line so that automation of the wiring work and cost reduction could be achieved.
There exists a growing tendency in recent years to use the stepping motor as a prime mover of an analog instrument installed on a mobile unit, such as a motor vehicle. In such applications, a pointer 6 may be fixed to an extreme end of the drive shaft 1 of the stepping motor 3 with a dial plate 7 located at the back of the pointer 6 as shown by broken lines in FIG. 6. In this kind of analog instrument, an alternative construction as shown in FIG. 7, for example, is occasionally employed. Specifically, terminals 4 of a stepping motor 3 extend in the same direction as the output end of a drive shaft 1 and a circuit board 5 is provided on a front surface (top side in FIG. 7) of the stepping motor 3, where the output end of the drive shaft 1 projects, wherein the output end of the drive shaft 1 passes through the circuit board 5 and light-emitting diodes (LEDs) 8 are mounted on the circuit board 5 to illuminate a pointer 6 and a dial plate 7 through a diffuser plate 9. This alternative makes it possible to achieve high-luminosity illumination with a simple construction and to reduce the thickness of the instrument.
When the circuit board 5 is provided on the front surface of the stepping motor 3 as shown in FIG. 7, however, it is not possible to use an automatic soldering process based on flow or reflow soldering technique on a production line, because the output end of the drive shaft 1 projects beyond the circuit board 5. Thus, this alternative construction inevitably requires manual soldering, resulting in a low labor efficiency in electrical wiring and an increase in production costs. A further problem is that the construction of a main motor unit of the stepping motor 3 varies depending on whether the circuit board 5 is located at the front surface or at the rear surface of the stepping motor 3 due to the difference in the structure of the terminals 4 so that it is impossible to use a common main motor unit for the aforementioned two instrument constructions.